1. Field of the Invention
The invention relates to a light-modulating cell, particularly for variable reflection of light, variable transmission of light, display of signals and images such as alphanumerical, graphical and other optical information. The invention applies in particular to various electro-optical devices, in particular, display panels, variable transparency windows, shop windows, screens, windscreens, spectacles, light valves, shutters, variable reflection mirrors, memories, and so forth.
2. Related Art
Numerous processes and devices for light modulation are known. Among those of particular interest are those enabling the production of electro-optical devices which are very thin with respect to their area, in particular with regard to display devices and in particular flat screens of so-called liquid crystals, electrochromic and electrophoretic type.
Among these various light-modulating techniques, the electrochromic processes use the reversible change of color and/or of optical density obtained by the electro-chemical oxidoreduction of a so-called electrochromic material whose oxidized form and whose reduced form have different colors and/or optical densities.
Electrochromic light-modulating processes have characteristics which are noteworthy for numerous applications: low control voltage (having a maximum of a few volts); low energy consumption; open circuit (nonvolatile) memory; and relatively uncritical distance requirements between electrode and counter-electrode. They also have other characteristics which are particularly advantageous for display devices: very high contrast even when viewed laterally at a high angle; excellent visibility by reflection under high-illumination conditions such as in bright sunshine; extended grey scale; and wide operating temperature range (often extending to low temperatures).
The low control voltage enables the use of low cost electronic control and addressing means. Furthermore, low energy consumption enables applications where independent operation (on batteries or accumulators) is required.
However, known processes and electrochromic devices have a certain number of disadvantages which limit their fields of application.
In general, elementary cells of known electrochromic light modulating devices are sealed (individually or in combination with other cells) in a way which is strictly leaktight with respect to the external ambient atmosphere. Known cells generally comprise (1) a transparent front electrode deposited on (2) a transparent plate of glass or plastic material, (3) an electrochromic material (often in the form of a thin layer deposited on the transparent electrode), (4) a gap, filled with electrolyte, (5) a counter-electrode (also transparent if the device functions by transmission), and (6) conductors for electrical connection of each electrode to an electronic control means external to the cell. Known cells also most often comprise a specific separator intended to maintain between the electrode and the counter-electrode an electrolyte-filled gap of constant thickness. Known cells also comprise structural means employing material and seals intended to maintain cohesion and permanence of internal physical and electrical contacts which are necessary for correct operation. At least the front electrode and/or the layer of electrochromic material are delimited in such a way as to define the shape required for the corresponding picture element (such as image point or image segment).
A strictly leak-tight sealing is necessary to prevent loss (particularly by leakage or evaporation) of constituents of the internal medium, particularly constituents of the electrolyte. Leak-tight sealing is also necessary to prevent the entry into the cell of constituents of external ambient atmosphere (for example, oxygen, carbon dioxide, humidity, and various contaminating substances) which are often capable, even in traces, of altering or degrading the constituents of the internal medium, of introducing parasitic processes, of affecting the operation of the cell, and of reducing its lifetime.
The sealing problem is a significant problem at points where the cell must provide a sealed passage for the conductors connecting the front electrode and the counterelectrode to the external electronic means. The seals, which must be compatible with the various materials used, are subject to mechanical stresses resulting in particular from differences between the coefficients of expansion of these materials.
This sealing problem is aggravated when the dimensions of the device are increased. Stresses of thermal origin can increase because of asymmetry in exposure to heat sources. Stresses of mechanical origin occur, due to vibrations to which a panel of large dimensions is naturally exposed. Interaction with the structure for mounting and holding the panel also introduce stresses.
The necessity of such a strictly leak-tight sealing, and the problems which it raises, are explicitly mentioned and justified by numerous documents, with respect to electrochromic materials, electrolytes and various structures. In particular, reference is made to U.S. Pat. No. 4,127,853; FR 83,041,162 (cell containing a metallic oxide as an electrochromic material and a liquid organic electrolyte from which the molecular oxygen must be removed); FR 7,443,548 (for several classes of solid electrolytes, necessary support-- using a sealed casing-- of particular conditions of humidity, pressure, vacuum or gaseous atmosphere essential for the correct operation of the device); U.S. Pat. No. 4,128,315 (sealing necessary to prevent loss of humidity); U.S. Pat. No. 4,116,546 (use of a solid electrolyte for the particular purpose of avoiding rapid degradation of the seal observed with liquid or acidic semi-solid electrolytes); U.S. Pat. No. 4,167,309 (protection from atmospheric oxygen of radical type electrochromic materials); U.S. Pat. No. 3,704,057 (seal for sealing a cell containing tungsten trioxide as an electrochromic material and a semi-solid gelled electrolyte); U.S. Pat. No. 3,708,220 (cell preventing any leakage by self-sealing of the electrolyte inlet orifice); J. Duchene et al, IEEE Transactions on Electron Devices, Vol. RD-26, No. 8, August 1986, p. 1263 (electro-deposited cell with organic liquid electrolyte sealed by a sealing glass).
In known electrochromic processes and devices, there are several types of electrochromic materials and generation erasure mechanisms of optical density and/or of coloring, each having its own problems which add to the problems described above. These problems include the following:
(1) Oxidoreduction of non-stoechiometric electrochromic solids. A considerable number of electrochromic solids have been used, which are generally solids which are insoluble in the two states of oxidation between which they change color; these solids are electrically insulating or slightly conducting. Among inorganic materials the following can be particularly mentioned among others: WO.sub.3, MoO.sub.3, V.sub.2 O.sub.5, Nb.sub.2 O.sub.5, IrO.sub.x. (An extensive list is given, for example, in U.S. Pat. No. 3,704,057.) Among organic materials are diphthalocyanine of Lu, and of Yb in particular.
These electrochromic solids must generally be used by depositing a thin layer on the transparent electrode by means of costly vacuum deposition techniques (evaporation under vacuum, cathodic sputtering in particular). Their change of color is generally from colorless or from a primary color to a second different color: colorless to blue for WO.sub.3 and MoO.sub.3, yellow to green for V.sub.2 O.sub.5, colorless to blue or blue-black for IrO.sub.x, green to red for diphthalocyanin of lutecium.
The most-used of these electrochromic solids, tungsten trioxide WO.sub.3, has problems, in addition to those already mentioned, which are representative of those of this class of electrochromic materials: very high sensitivity to contaminating substances, particularly atmospheric (document FR 83,041,162), degradation by corrosion with dissolution in the aqueous and polymeric electrolytes (U.S. Pat. No. 4,215,915, U.S. Pat. No. 3,970,365), reduced but not eliminated inorganic electrolytes (Kodintsev et al., Electrokhimiya 1983, Vol. 19, No 9, page 1137).
Complex techniques, for example oblique evaporation (U.S. Pat. No. 4,390,246), are required for improving the color generation and erasure characteristics which are very sensitive to slight changes in preparation and composition. In most display devices (for example, U.S. Pat. No. 4,128,315), the tungsten trioxide film must be deposited with a delimitation according to the shape and dimensions of the picture element (image segment or image point). Finally, the cells have neither the voltage threshold nor the memory in a circuit coupled to other cells which would be necessary for multiplexed matrix operation (Yoshiro Mori, J.E.E., August 1985, page 53).
(2) Oxidoreduction of radioal compounds. The most representative and most studied of the materials of this class is heptyl-viologen. Dissolved in the electrolyte where it is colorless, heptyl-viologen deposited by reduction is a blue or red colored film on the transparent electrode and is redissolved by oxidation (U.S. Pat. No. 4,116,535). But it is known that the deposit progressively recrystallizes in a form which cannot be redissolved, which severely limits the number of accessible cycles and the lifetime. Alternatively, the electrode passivates, considerably reducing the speed of the writing reaction for which it is then necessary to catalyze, for example, by depositing metallic ions (document EP 0,083,668). Finally, the cells do not have either a threshold or a memory in a circuit coupled to other cells.
(3) Electrodeposition of metals. The reversible electrodeposition of metals from an electrolytic solution has been the subject of various works, particularly with liquid organic electrolytes, because of corrosion problems and parasitic reactions harmful to the stability and lifetime encountered with aqueous electrolytes. For example, Y. Duchene et al. (in the article cited above), describes a display cell which uses as an electrolyte, methanol or acetonitrile containing silver iodide and sodium iodide. The silver ions reduce into a silver film having a high contrast. However, for a given electrical charge, the optical density depends on the current density used, and inhomogeneities appear on the deposited film after a certain number of deposition redissolution cycles. The cell does not have a writing voltage threshold and is not therefore suitable for multiplexed matrix writing. The zone of the transparent electrode corresponding to the display, must be delimited inside the cell by means of an insulating layer engraved according to the design of the zone in question. Finally, the use of a glass sealing technology is indicated as one of the conditions of reliability, confirming the importance of strictly leak-tight sealing.
A similar cell described by I. Camlibel et al. (Appl. Phys. Letters 33,9, Nov. 78, page 793) contains silver iodide and potassium iodide in dimethylsulphoxide, and produces a specular gilt or bright red deposit, depending on conditions.
(4) Electro-active polymers (redox). Recent works relate to polymers such as polyaniline, polyacetylene, polyrrole, and polythiophene, in particular which, in thin layer on a transparent electrode, can change color (for example from red to blue for polythiophene) depending on their state of oxidation. These materials, which are generally rather unstable or easily alterable, have a short lifetime and do not enable a very large number of operating cycles.
It has been seen that most known electrochromic cells do not have a definite electrical voltage threshold (i.e., an electrical voltage below which a picture element is not written). Furthermore, although most of these cells have an open circuit (nonvolatile) memory (i.e., a persistence of the written state when the electrical writing voltage is disconnected), this memory partially discharges if a written cell is connected to an erased cell, such that the first cell partially erases while the second partially writes. In this event, the optical density of the cells tends to become uniform with time. The absence of a definite writing threshold and/or a persistent memory in a circuit coupled to another erased cell, prohibit the matrix writing of a system of picture elements placed at the intersections of two orthogonal arrays of parallel conductors.
Analysis confirms that the non-selected picture elements are partially written while the selected picture elements are partially erased. The optical density of the selected picture elements and that of the nonselected picture elements approach each other, thus degrading contrast and even eliminating it.
In known systems, it is exceptional to obtain a genuine black in the written state. It is also uncommon to obtain a genuinely white or colorless transparent appearance in the erased state. Generally, colors such as blue, blue-black, purple, and so forth, are obtained. Apart from the aesthetic preference for a color or for black, the production of a particular color prohibits a multi-color display by a three-color process (unless it becomes possible to generate the three primary colors). On the other hand, the production of a genuine black in the written state and a genuine white in the erased state (or a colorless transparent appearance in transmission) enables multicolor display by additive synthesis by associating picture elements with blue, green and red colored screens or filters according to a repetitive distribution.
Numerous known electrochromic devices use a liquid electrolyte (for example, an aqueous electrolyte such as an aqueous solution of sulfuric acid (document FR 7,626,282), or an organic electrolyte such as a solution of lithium perchlorate in propylene carbonate (Yoshiro Mori article, cited above)). This electrolyte, which cannot generally be common to several cells for electrical reasons, requires individual confinement in each cell which must comprise an electrolytic compartment which must not be distorted. In addition to the problems raised by the individual filling and sealing of each cell, the particularly complex structure which is obtained, despite its cost, does not enable a high resolution display device (such as a computer screen). If it appears possible to reduce the size of the picture element to the necessary values (of the order of a few hundred microns), the size of the cell (and particularly the needed lateral walls), does not enable reduction of the gap between adjacent image-points to a value which should be of the order of a few tens of microns at most.
In order to reduce the complexity of the display cell brought about by the problems of confinement of liquid electrolyte, there has been used gelled semisolid liquid electrolytes (U.S. Pat. No. 3,708,220: gelled sulfuric acid), polymers with acidic functions (U.S. Pat. No. 4,116,545), and ion exchange membranes (U.S. Pat. No. 4,128,315). The structure of the cells is actually simplified, and in certain cases has the additional advantage of surface adhesion properties (tackiness), simplified construction, and viscoelastic properties which improve the contacts. But all of these electrolytes used in association with a layer of solid electrochromic material deposited on the transparent electrode contain, in one way or another, a certain quantity of water (by constitution, hydration, impregnation, and so forth). The cells have, to varying degrees, the corrosion problems mentioned above, as well as the necessity of a leak-tight sealing.
In view of avoiding the use of a free liquid electrolyte, inorganic solids have also been used which have ionic conductivity, such as for example beta alumina (M. Green et al., Solid State Ionics 3/4, 1981, pages 141 to 147, North-Holland), or polymers having ionic conduction such as, for example, solid solutions of lithium perchlorate in polyethylene oxide (document FR 8,309,886). However, it is well known that such solid electrolytes, at ambient or ordinary temperatures have only a generally very low ionic conductivity, considerably impeding the speed of writing and erasure which may require several seconds or even more. Furthermore, a progressive degradation of the electrical contact between the inorganic solid electrolytes and the electrodes is often observed. This degradation has a harmful effect on the lifetime of the light-modulating cells.
In known electrochromic devices, the counter-electrode is often of complex and expensive manufacture and structure because of the functions that it may have to simultaneously provide. The functions include the auxiliary redox function, maintaining a constant specified electrode potential, high charge capacity, reversibility, and so forth, while being capable of a high number of cycles without degradation. For example, a counter-electrode has been produced comprising a second layer of an electrochromic solid modified in such a way as to have a low electrochromicity and deposited on a transparent electrode (U.S. Pat. No. 4,278,329). Another known counter electrode is a sheet of paper formed with acrylic fibers, a binder and carbon powder, in which there is also incorporated an electrochromic solid (U.S. Pat. No. 4,088,395). Another counter-electrode whose electrode potential is adjustable comprises carbon powder, a binder and mixtures of depolarizers W.sub.18 O.sub.49 and V.sub.6 O.sub.13 in adjustable proportions (Yoshiro Mori article cited above).
The structure and manufacture of known electrochromic display screens are generally complex and expensive, especially when the size of the panel is large. Beyond a certain size, technical problems and manufacturing costs become such that large display panels car only be produced in the form of a mosaic of small independent panels.
There is also known (in document FR 2,504,290) a process for recording signals and images in which:
(1) A recording medium is formed, comprising at least an electrochromic material having at least one free surface constituted by a mixture of solid consistency of at least (a) a water-soluble salt or a water-soluble mixture of salts of at least one metal which can be cathodically deposited from an aqueous solution of one of its ions; and (b) an initially water-soluble film-forming polymer resin, preferably in the proportion of 1 part by weight to 0.5 to 50 parts of anhydrous salts; and (c) water;
(2) There is placed in contact with the free surface of the electrochromic material, at a place where it is desired to form a mark, an electrode, taken, with respect to the said material, to a negative potential in order to make an electrical current flow between the electrode and the material;
(3) There is formed in the electrochromic material, in the zone of contact with the electrode, a mark which is immediately and directly visible by cathodic reduction of at least one depositable metallic ion and is present in the material, and at least one metal which electrocrystallizes and is an integral part of the material, the metal constituting the mark.
According to this document, the electrochromic material, and the process for its implementation, intend to obtain a mark (signal or image) which is essentially stable in time.
On the other hand, this document is not interested in, does not suggest and does not describe the application of such an electrochromic material for modulating light nor a corresponding implementation process.